1. Field of Invention
The invention relates to a high-pressure fuel supply system of an internal combustion engine, and a control method thereof.
2. Description of Related Art
In order to inject fuel directly into cylinders of an internal combustion engine, high-pressure fuel must be supplied to each fuel injection valve. A high-pressure fuel supply system for this purpose is known in the art.
A conventional high-pressure fuel supply system includes a delivery pipe leading to each fuel injection valve, a high-pressure pump for force-feeding high-pressure fuel into the delivery pipe, and a low-pressure pump connected to the intake side of the high-pressure pump in order to ensure fuel intake of the high-pressure pump.
When operating an engine, the high-pressure pump force-feeds fuel. This enables the pressure in the delivery pipe to be held at a value close to a desired high fuel pressure suitable for satisfactory fuel injection. However, the high-pressure pump is commonly an engine-driven pump. Therefore, the high-pressure pump cannot force-feed a sufficient amount of fuel at an extremely low engine speed in a cranking period upon starting of the engine. As a result, it takes a very long time to raise the pressure in the delivery pipe to the desired high fuel pressure.
It is impractical to spend such a long time upon starting the engine. Therefore, at the risk of impeding satisfactory fuel injection, fuel injection is commonly started when the fuel pressure in the delivery pipe is lower than the desired high fuel pressure.
Japanese Patent Laid-Open Publication No. 2000-8917 proposes the following method: when the fuel pressure in a delivery pipe is lower than a prescribed pressure upon starting an engine, fuel discharged from a low-pressure pump is introduced into the delivery pipe through a high-pressure pump by, e.g., keeping a spill valve of the high-pressure pump in the open state. Fuel injection is thus started at a fuel pressure (in the delivery pipe) equal to a rated discharge pressure of the low-pressure pump. On the other hand, when the fuel pressure in the delivery pipe is equal to or higher than the prescribed pressure, the delivery pipe is compressed by the high-pressure pump, and fuel injection is started at a fuel pressure higher than the rated discharge pressure of the low-pressure pump.
When the engine is started at an extremely low temperature, injected fuel is less likely to be evaporated. In order to assure reliable ignition, a large amount of fuel must be injected at least in the first fuel injection to each cylinder on the assumption that only a part of the injected fuel is evaporated.
A low-pressure pump is commonly an electrically driven pump. Therefore, it is possible to force-feed a relatively large amount of fuel from the time the engine is started. Accordingly, in the case where fuel injection is started at a pressure (in the delivery pipe) equal to the rated discharge pressure of the low-pressure pump in the high-pressure fuel supply system described in the above Japanese Patent Laid-Open Publication No. 2000-8917, a sufficient amount of fuel is discharged from the low-pressure pump and supplied to the delivery pipe right after the fuel is injected to the first cylinder. As a result, the pressure in the delivery pipe immediately reaches the rated discharge pressure of the low-pressure pump even if a large amount of fuel is injected as in the case where the engine is started at an extremely low temperature. Accordingly, fuel injection to the following cylinder can be conducted at the rated discharge pressure of the low-pressure pump. However, even if the fuel is injected at the rated discharge pressure of the low-pressure pump, it is difficult to evaporate an intended amount of fuel within the cylinder due to a low injection penetration, i.,e. a long distance over which the fuel injected remains in a liquid state. As a result, it is difficult to implement satisfactory starting of the engine.
On the other hand, in the case where the fuel in the delivery pipe is compressed by the high-pressure pump and fuel injection is started at a pressure higher than the rated discharge pressure of the low-pressure pump, fuel injection to the first cylinder can be conducted in a relatively satisfactory manner upon starting the engine at an extremely low temperature. However, in a case where the engine is started at an extremely low temperature, a large amount of fuel is injected to the first cylinder. In such a case, the fuel pressure in the delivery pipe becomes lower than the rated discharge pressure of the low-pressure pump after the fuel injection to the first cylinder, thereby making fuel injection to the following cylinder difficult. Moreover, the high-pressure pump has a small size. Therefore, even if the high-pressure pump force-feeds the fuel to the delivery pipe before the fuel injection to the following cylinder, the amount of fuel discharged from the high-pressure pump in each discharge operation is often smaller than the amount of fuel injected to each cylinder when the engine is started at an extremely low temperature. Therefore, even if the fuel injection to the following cylinder is conducted at a pressure higher than the rated discharge pressure of the low-pressure pump, it is difficult to inject the fuel to all of the remaining cylinders at a fuel pressure higher than the rated discharge pressure of the low-pressure pump. Accordingly, it is also difficult to implement satisfactory starting of the engine.
The invention thus provides a high-pressure fuel supply system of an internal combustion engine and a control method thereof, which enable implementation of a satisfactory starting of the engine.
A first aspect of the invention relates to a high-pressure fuel supply system of an internal combustion engine. The high-pressure fuel supply system includes a high-pressure portion for supplying fuel to a plurality of fuel injection valves, a high-pressure pump for supplying fuel to the high-pressure portion, and a low-pressure pump for supplying fuel to the high-pressure pump. Upon starting the engine, the high-pressure fuel supply system starts fuel injection after a pressure in the high-pressure portion is raised to a preset pressure, higher than a rated discharge pressure of the low-pressure pump, by the high-pressure pump. Furthermore in the high-pressure fuel supply system, in view of a fuel consumption amount consumed from the high-pressure portion through the fuel injection valve and a fuel supply amount supplied to the high-pressure portion by the high-pressure pump, the preset pressure at which the fuel injection is started is set so that first fuel injection to each cylinder of a plurality of cylinders by each of the plurality of fuel injection valves is conducted at a pressure higher than the rated discharge pressure of the low-pressure pump.
A second aspect of the invention relates to a method for controlling a high-pressure fuel supply system of an internal combustion engine. The high-pressure fuel supply system includes a high-pressure portion that supplies fuel to a plurality of fuel injection valves, a high-pressure pump for supplying fuel to the high-pressure portion, and a low-pressure pump for supplying fuel to the high-pressure pump. The method includes the steps of calculating a fuel consumption amount consumed from the high-pressure portion through the fuel injection valve; calculating a fuel supply amount supplied to the high-pressure portion by the high-pressure pump; and setting a preset pressure in the high-pressure portion, in view of the fuel consumption amount and the fuel supply amount, at which fuel injection is started so that first fuel injection to each cylinder of a plurality of cylinders by each of the plurality of fuel injection valves is conducted at a pressure higher than a rated discharge pressure of the low-pressure pump.